A positive photoresist generally uses an alkali-soluble resin, mixed with a naphthoquinonediazide compound as a dissolution inhibitor. The alkali-soluble resin is generally novolac resin. A positive photoresist using the novolac resin is useful because it can be developed with an alkaline aqueous solution without swelling, which provides very good resolution, and especially because it has a high resistance to plasma etching when the resulting image is used as an etching mask. With increasing integration and fine patterns due to recent advance in semiconductor technology, photoresists are required to have resolutions of one-half micron or better.
Currently, in the formation of fine patterns used in various semiconductor integrated circuits and magnetic valves, the photoresist is exposed mainly by reduced projection exposure. The resolution R in reduced projection exposure follows Rayleigh's Equation (1): ##EQU1## There are three methods to improve this resolution: (1) numerical aperture NA of the lens is increased, (2) wavelength (.lambda.) of exposure light is decreased, and (3) constant k, which is determined by the resist process, is reduced.
Heretofore, improvement of resolution has been predominantly achieved by increasing the NA of the stepper and improving the resist process. However, increasing NA of the lens also decreases the depth of focus (DOF), in accordance with Equation (2): ##EQU2##
On the other hand, since decreasing the wavelength .lambda. has much smaller influence on DOF than increasing NA, reduced projection exposure devices which use the shorter wavelength i-line (365 nm wavelength) of a mercury lamp, rather than the g-line (436 nm wavelength) of a mercury lamp in the prior art systems, along with photoresists for use with these devices have recently been developed. Currently, commercial i-line photoresists use predominantly phenol esters of 1,2-naphthoquinonediazide-4-sulfonic acid or 1,2-naphthoquinonediazide-5-sulfonic acid as dissolution inhibitors, which are the same as those used for g-line photoresists. However, the above i-line photoresists provide insufficient resolution, and the resulting resist patterns are not satisfactory.
This is because, when used for i-line radiation (compared with the use for g-line radiation), the dissolution inhibitor, the above-mentioned phenol esters are low in transmissivity, so that it is difficult for light to reach the bottom of the photoresist, resulting in large differences in the amounts of exposure between the top and bottom of the photoresist.
Therefore, if the content of the dissolution inhibitor is reduced to increase the transmissivity of the photoresist to i-line radiation, the difference in solubility of the developing solution between the exposed portion and unexposed portion of the photoresist tends to become small, resulting in film loss.
Another problem of prior art positive photoresists is a low sensitivity to long-wavelength light such as that generated by an argon laser or the like. For example, in the fabrication of an optical disk base plate, in which a positive photoresist coated on a glass disk is exposed to an argon ion laser to form bits, the exposure requires a long time because the prior art photoresist has almost no absorption at the 488 nm wavelength generated by the argon ion laser.
Recently, the details of the photoreaction mechanism of naphthoquinonediazide-based photosensitive agents have been well elucidated, and the correlation between the photosensitivity characteristics of photoresist and its performance has been clarified (e.g. "Ultra-Fine Processing and Resist Materials" 1985, published by CMC K. K.). Parameters indicative of the photosensitivity characteristics include photoreaction parameters A and B, which are values related to the ratio of T (transmissivity), proposed by Dill et al. (Dill et al., IEEE, Trans. B. D., Vol.22, No.7, p.445).
As shown in the following Equations (3) and (4), A represents the ratio of transmissivities before and after exposure, and B represents the transmissivity after exposure EQU A=(1/d)l.sub.n [T(.infin.)] (3) EQU B=-(1/d)l.sub.n T(.infin.) (4)
wherein d is the film thickness of the photoresist, T(o) is the transmissivity of the photoresist before exposure, and T(.infin.) is the transmissivity of the photoresist after exposure.
It has been known that the resist performance is varied by varying parameters A and B which are indicative of the photosensitivity characteristics of photoresist. Therefore, to design a high-resolution photoresist, it is necessary to optimize parameters A and B according to the intended application.
However, for 1,2-naphthoquinone-(2)-diazide-5-sulfonic acid or 1,2-naphthoquinone-(2)-diazide-4-sulfonic acid and phenols alone, which are photosensitive agents used for prior art photoresists, the only way to vary the parameters A and B of the photoresist, especially the value of the former, at g-line (436 nm) or i-line (365 nm) is to vary the ratio of the amounts of photosensitive agents, that is, the ratio of the photosensitive agent to the alkali-soluble resin. However, increasing the amount of the photosensitive agent increases the value of parameter A, but decreases the sensitivity, and decreasing the amount of the photosensitive agent decreases the value of parameter A, but tends to cause a film loss.